The invention relates to a high-frequency low-pass filter designed especially for radiocommunication equipment used in mobile communication networks.
Low-pass filters in current and future mobile communication networks have to have true low-pass characteristics, i.e. their stop-band attenuation must remain relatively high up to at least 10 GHz. Good breakdown characteristics and power capacity are also often required of the filters. For example, in wideband code division multiple access (WCDMA) equipment the strength of the electric field of the transmission signal has momentary peaks that may cause breakdown in an insulator. Severe demands may be imposed on the power capacity of a low-pass filter especially in cases where several transmission signals are summed together. At least a desirable characteristic in most filters is that they have small losses. Small losses mean low attenuation on the pass band and easier matching. Moreover, as regards massproduced filters that must meet certain requirements, the question of production costs is of essence.
From the prior art it is known high-frequency low-pass filters constructed of coils and capacitors. These are useable at frequencies of up to a few GHz. At frequencies higher than that the characteristics of filters realized using said components become highly degraded due to parasitic effects in the components. The self resonances of coils and capacitors may result in considerable variation in the stop-band attenuation at high frequencies. This drawback can be alleviated by realizing the low-pass filter as a series connection of two low-pass filters such that the first filter attenuates in a certain frequency band and the second filter attenuates in a frequency band higher than that. The drawback in that case is that it affects production: the construction uses components that have very low capacitance or inductance values, and further the tolerances of these values are small.
The drawbacks caused by the self resonances of components can be alleviated by using distributed capacitance and inductance circuit elements instead of capacitors and coils. Such elements are often realized by etching them on the surface of a printed circuit board. FIG. 1a shows an example of a prior-art low-pass filter formed on a printed circuit board. The filter comprises conductive areas, such as 11, 12, 13 and 14, arranged in series on a printed circuit board 10 and a ground plane that may be a metal plating on the opposite surface of the circuit board or a protective housing around the circuit board. The feed line of the filter is connected between the input end IN of the conductive areas and ground, and the signal is taken out from between the output end OUT of the conductive areas and ground. Every other conductive area, such as 11 and 13, is relatively wide. What is essential in them is their capacitance in relation to the ground plane. Every other conductive area, such as 12 and 14, is relatively narrow. What is essential in them is their inductance. The equivalent circuit of the filter is thus in accordance with FIG. 1b. Starting from the input end it comprises inductances L1-L8 connected in series. From between the inductances capacitances C1-C7 are connected to ground. The values of the inductances and capacitances naturally depend on the dimensions of the conductive areas, which thus determine the filter""s response. In practice, the filter has two parts such that the part corresponding to inductances L1-L4 and capacitances C1-C4 together with the impedance of the feeding port attenuate sufficiently from a desired cut-off frequency to a second frequency. The parts corresponding to inductances L5-L8 and capacitances C5-C7 together with the impedance of the circuit fed by the output attenuate sufficiently from said second frequency to an even higher third frequency. If the cut-off frequency of the filter in FIG. 1 is of the order of one gigahertz, the structure is drawn enlarged. The length of the individual parts in a conductive area on the printed circuit board in the direction of signal propagation is very small compared to signal wavelength. At frequencies one order of magnitude higher than the signal frequency the circuit elements should be viewed as transmission lines. Indeed the two-part nature of the filter is due to the fact that at high frequencies the first part of the filter produces transmission line resonances which decrease the stop-band attenuation.
Low-pass filters implemented on printed circuit boards are highly suitable for series production. Their drawback is that in high-power applications the power capacity of the circuit elements may prove insufficient. Another drawback is that in demanding applications the losses caused by the circuit board on the signal transferred may be too high. Still another drawback is that when feeding multiple high-frequency signals into a low-pass filter implemented on a printed circuit board, the nickel used on top of the copper in a conductive area may cause harmful intermodulation products.
An object of the invention is to reduce said disadvantages of the prior art. The filter construction according to the invention is characterized by what is expressed in the independent claim. The dependent claims disclose preferred embodiments of the invention.
The basic idea of the invention is as follows: The low-pass filter uses distributed capacitance and inductance elements. These are realized using a homogeneous and relatively thick conductive part comprising alternate inductive and capacitive elements in series. The conductive part is coated with silver, for example, and it is located mainly air-insulated in an enclosed metal housing that serves as a signal ground conductor and as a protective shield against interfering fields. The housing may have conductive partition walls in order to prevent coupling between adjacent capacitive elements. The conductive part, which forms the core of the filter, is supported to the housing through dielectric material. The ends of the housing have through holes for the input and output lines of the filter.
An advantage of the invention is that a filter according to the invention has a good power handling capacity because the conductors have relatively large cross sectional surfaces. Another advantage of the invention is that the losses of the filter are relatively low because the elements are air insulated and have relatively large cross sectional surfaces. A further advantage of the invention is that the construction according to the invention causes relatively little intermodulation because it does not use ferromagnetic coating materials and there are only a few conductor junctions. Yet another advantage of the invention is that the filter has stable characteristics. Furthermore, an advantage of the invention is that the manufacturing costs of a filter that meets certain attenuation requirements are relatively low because of the simple construction.